Abstract : This paper is concerned with the study of the correlation of the rate of uptake of I131 in the thyroid with the clinic ci evaluation of thyroid function as determined by history, physical examination and other laboratory procedures. It has become conventional to study the concentration of 1131 in the thyroid as a function of time and to relate this to the clinical estimate of thyroid function. The maximum concentration and the shape of the concentration curve obtained over a period of several days have been used, with clinical and laboratory evidence from the study of patients, to determine the need for definitive treatment of the patient suspected of having hyperthyroidism. These studies are time-consuming. At the suggestion of A.V.H in the Spring of 1949, an attempt was made to find a single, more quickly thyroidism These studies are time-consuming. At the suggestion of A.V.F. in the Spring of 1949, an attempt was made to find a single, more quickly obtained value from the uptake curve that might be as useful as the entire uptake curve in differentiating the normal from the hyperthyroid patient. The rate of uptake of I131 in the thyroid during the interval of a few hours after its administration to the fasting patient, expressed as percent-per-hour, was tried first. Most data were collected en patients around 3 to 6 hours after the administration of the I131.It was realized that the "%/hr" has meaning only if it is used as an average rate of concentration or if the concentration curve is linear. It seemed from the study of curves of concentration available to use that these curves were near enough to a straight line during the early hours to intake such an observation have meaning.